Material based on vinylester resin for neutron shielding and maintenance of sub-criticality

ABSTRACT

This invention relates to a composite material for neutron shielding and maintenance of sub-criticality comprising a matrix based on vinylester resin and an inorganic filler capable of slowing and absorbing neutrons. 
     The vinylester resin may be an epoxymethacrylate resin and the inorganic filler may contain a zinc borate and an alumina hydrate or magnesium hydroxide.

TECHNICAL DOMAIN

The purpose of this invention is a material for neutron shielding andmaintenance of sub-criticality. This type of material is useful innuclear energy to protect operators from neutron radiation emitted byradioactive products and to prevent runaway of the neutron formationchain reaction, more particularly when these products contain fissilematerials.

In particular, they can be used as neutron shielding in transportpackagings and/or for the storage of radioactive products, for examplenuclear fuel assemblies.

For neutron shielding, neutrons have to be slowed down and thereforematerials containing large quantities of hydrogen have to be used,including the addition of a boron compound to capture neutrons.

To maintain sub-criticality, it is necessary to have a high content ofneutron absorber such as boron to prevent runaway of the neutronformation chain reaction.

Moreover, these materials must be self-extinguishing.

STATE OF THE PRIOR ART

Neutron shielding materials obtained from a mix of a high-densityinorganic material and a thermosetting resin have been described inEP-A-0 628 968 [1]. In this document, the thermosetting resin may be anunsaturated polyester resin and the inorganic fillers may be heavymetals or compounds of heavy metals.

Document GB-A-1 049 890 [2] describes moulded articles or coatingsabsorbing neutrons containing at least 0.3% by weight of boron obtainedfrom a co-polymerisable mix of an unsaturated polyester and anunsaturated monomer, in which either the acid component of the polyesteris derived partly from boric acid, or the polymerisable monomer ispartly a boric acid ester.

Document JP-A-55 119099 [3] describes materials providing protectionagainst neutrons also based on unsaturated polyester resin. This type ofmaterial has a hydrogen atoms density equal to 6.1×10²² atoms ofhydrogen per cm³, but it does not contain any neutron absorber. Thus, itcannot keep a nuclear fuel transport packaging sub-critical.

These materials based on unsaturated polyester resin have thedisadvantage that they have only a mediocre resistance to thermal aging.

PRESENTATION OF THE INVENTION

The purpose of this invention is specifically a material for neutronshielding and maintenance of sub-criticality that has better resistanceto corrosion than materials based on unsaturated polyester.

According to the invention, the composite material for neutron shieldingand maintenance of sub-criticality comprises a matrix based onvinylester resin and an inorganic filler capable of slowing andabsorbing neutrons.

According to the invention, the vinylester resin may be of differenttypes. In general, the resins used are obtained by the addition of acarboxylic acid onto an epoxy resin.

The epoxy resins used have one of two possible types of macromolecularpattern:

-   -   bisphenol A, and    -   novolac.

In particular, the carboxylic acid may be acrylic acid or methacrylicacid. Preferably, methacrylic acid is used.

Thus, the vinylester resin is preferably chosen from the group composedof epoxyacrylate resins, epoxymethacrylate resins, bisphenol A typeresins, novolac type resins and halogenated resins based on bisphenol A.

The epoxyacrylate and epoxymethacrylate bisphenol A type resins maycomply with the following formula:

in which R represents H or CH₃.

The novolac type vinylester resins may comply with the followingformula:

-   -   in which R represents H or CH3.

Halogenated vinylester resins based on bisphenol A may also be usedaccording to the invention, for example complying with the followingformula:

in which R is as defined above.

Non-epoxy vinylester resins may also be used in the invention, obtainedfrom isophthalic polyester and urethane, for example complying with thefollowing formula:

in which R is as defined above and U represents a urethane group.

Due to the choice of these vinylester resins, the composite materialaccording to the invention has the following advantages.

The atomic concentration of hydrogen in vinylester resins is greaterthan the atomic concentration of unsaturated polyesters, thereforeneutron slowing is better.

These resins have excellent thermal stability and a very good resistanceto corrosion, which is advantageous for materials used for neutronshielding and maintenance of sub-criticality, for which usagetemperatures are often high.

The material is easy to make since the vinylester resin may be poureddirectly into the mould that will form the transport or storagepackaging for radioactive products.

The loss of mass of shielding materials made of these vinylester resinsis low at high temperature.

In the material according to the invention, the vinylester resins havebeen transformed into a thermosetting material by reaction with acopolymerisable monomer such as styrene and styrene derivatives such asmethylstyrene and divinylbenzene, vinyltoluene, methyl methacrylate andallylic derivatives such as diallyl phthalate.

According to the invention, the material also comprises an inorganicfiller capable of slowing down and absorbing neutrons, for examplemetals, metal compounds, boron, boron compounds.

According to the invention, this inorganic filler may in particularcomprise at least one inorganic compound of boron and at least onehydrogenated inorganic compound. Boron compounds that could be usedbelong to the group comprising boric acid H₃BO₃ , colemaniteCa₂O₁₄B₆H₁₀, zinc borates Zn₂O_(14.5)H₇B₆, Zn₄O₈B₂H₂ and Zn₂O₁₁B₆, boroncarbide B₄C, boron nitride BN and boron oxide B₂O₃. Preferably, thecomposite material according to the invention comprises at least oneboron compound chosen from among zinc borate Zn₂O_(14.5)H₇B₆ and boroncarbide B₄C.

The hydrogenated inorganic compounds that could be used belongpreferably to the group of alumina hydrates and magnesium hydroxide.

The material according to the invention may also include polyvinylacetate, to make the material non-shrinking.

This material may also comprise a hydrogenated organic filler such asmelamine, to improve its self-extinguishing properties.

According to the invention, it is preferable to choose the inorganiccompound of boron and the inorganic hydrogenated compound and theirquantities so as to obtain a boron concentration in the material equalto 8×10²⁰ to 15×10²¹ of boron atoms per cm³ and a hydrogen concentrationof 4×10²² to 6×10²² atoms per cm³.

In the material according to the invention, the quantities of thedifferent constituents are also chosen to obtain density,self-extinguishing and thermal conductivity characteristics suitable foruse in a transport and/or storage packaging for radioactive materials.

In particular, it is necessary to have good resistance to aging at arelatively high temperature, since products put in the packaging mayreach a temperature of 170° C.

The material also needs to be fire resistant, which means that it shouldbe self-extinguishing, in other words the fire goes out when the flameis removed; and therefore it does not feed the fire.

According to the invention, this self-extinguishing property isconferred particularly by the presence of hydrogenated and/or boratedinorganic compounds, for example alumina hydrate or zinc borate.

Similarly, the material should have a low thermal conductivity, butsufficiently high to evacuate heat from transported elements such asirradiated fuel elements.

Finally, as will be seen later, since this material is obtained bypouring a mix of different constituents and a vinyl thinner, it isimportant that the quantities of the different constituents should besuch that the mix has the property that it can be poured. In general,the viscosity of the mix must not exceed 300 Poises.

As an example of a material composition according to the invention,consider the material containing 25 to 40% by weight of thermosettingvinylester resin, in other words including the vinyl thinner, forexample styrene.

Preferably, according to the invention, the density of the material isequal to or greater than 1.6, for example 1.65 to 1.9.

Preferably, the materials according to the invention can resist aminimum usage temperature of 160° C.

The material according to the invention may be prepared by setting a mixof constituents in the vinylester resin in solution in a vinyl thinner.

Thus, another purpose of the invention is a process for preparation ofthe composite material described above, which includes the followingsteps:

-   -   prepare a mix of vinylester resin in solution in a vinyl thinner        with the inorganic filler,    -   add a catalyst and an accelerator for hardening to the mix,    -   degas the mix under a vacuum,    -   pour the resulting mix in a mould, and    -   allow it to set in the mould.

The vinyl thinner may for example be styrene, vinyltoluene,divinylbenzene, methylstyrene, methyl acrylate, methyl methacrylate oran allylic derivative such as diallyl phthalate. Preferably, styrenewill be used which can both dissolve the vinylester resin and enablesetting by copolymerisation.

The catalysts and accelerators for hardening used are chosen from amongcompounds normally used for setting of vinylester resins. In particular,catalysts may be organic peroxides, for example:

-   -   peroxides derived from ketones, such as methylethylketone        peroxide, acetylacetone peroxide, methylisobutylketone peroxide,        cyclohexanone peroxide and cumen hydroperoxide;    -   diacyl peroxides, for example benzoyl peroxide, possibly        combined with aromatic tertiary amines such as dimethylaniline,        diethylaniline and dimethylparatoluidine; and    -   dialkyl peroxides such as dicumyl peroxide and ditertiobutyl        peroxide.

The most frequently used accelerators are divalent cobalt salts such ascobalt napththenate or octoate, and aromatic tertiary amines such asdimethylaniline, dimethylparatoluidine and diethylaniline.

One or more additives such as cross-linking inhibitors, surfactants andnon-shrinking agents can also be added to the mix.

Examples of inhibitors that could be used include acetylacetone andtertiobutylcatechol.

The method according to the invention is implemented as follows:

The vinylester resin (prepolymer+vinyl thinner) is mixed at ambienttemperature with the accelerator(s) and different inorganic fillers, forexample hydrogenated and borated fillers. The percentage of fillers mayvary from 60 to 75%. These fillers may also provide fire reactionproperties. The assembly is mixed so as to obtain a perfectly homogenousmix. The catalyst is added to the mix last. The homogenous mix is thendegassed under a vacuum (less than 0.01 MPa). The viscosity of the mixmust not exceed 300 Poises (the mix must be pourable).

After degassing, the mix is poured in the required mould in which it iscross-linked to form an insoluble thermosetting material. The mechanismof the reaction is radicalar and the reaction is highly exothermal. Thesetting time may vary depending on pouring conditions (temperature,catalyst content, accelerator and inhibitor contents). Thus, the geltime may be adjusted by varying the percentages of catalyst andaccelerators. The gel time varies from 20 minutes to 2 hours.

According to the invention, the mould used for setting of the resin maybe formed directly by the transport and/or storage packaging forradioactive products. For example, the packaging may comprise peripheralhousings in which the mix is poured.

Another purpose of the invention is a transport and/or storage packagingfor radioactive products comprising a shield formed from the compositematerial described above.

Other characteristics and advantages of the invention will becomeclearer after reading the following description of exemplary embodimentsobviously given for illustrative purposes and that are in no waylimitative, with reference to the appended drawing.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows mass losses (in %) at 160 and 170° C. of two materialsaccording to the invention as a function of time (in days).

DETAILED PRESENTATION OF EMBODIMENTS

The following examples illustrate the production of composite materialsfor neutron shielding and maintenance of sub-criticality, containingzinc borate and alumina hydrate or magnesium hydroxide using the resinmarketed by Dow Chemical under the trade name Derakane Momentum 470-300as the vinylester resin.

EXAMPLE 1

A polymerisable mix is prepared from Derakane Momentum 470-300vinylester resin, styrene, zinc borate Zn₂O_(14,5)H₇B₆ and magnesiumhydroxide using the proportions given in table 1 in the appendix.

The following constituents are added to the mix:

-   -   1% by weight, relative to the mass of resin+styrene, of the        55028 accelerator marketed by Akzo, and    -   2% by weight relative to the mass of resin+styrene, of the        Butanox M50 catalyst (methylethyl cetone peroxide) marketed by        Akzo.

The next step is vacuum degassing of the mix for 3 minutes followed bypouring the mix into a mould composed of a compartment of a nuclear fueltransport or storage packaging.

The gel time is 22 minutes at 20° C.

The result is a composite material with the following properties:

-   -   density: 1.697    -   hydrogen content: 4.72% by weight, namely 4.78×10²² atoms/cm³,    -   boron content: 0.97% by weight, namely 9.17×10²⁰ atoms/cm³.

The material obtained has satisfactory thermal properties.

The thermal coefficient of expansion α measured by TMA 40 (METTLER) witha temperature rise of 10° C./minute gives the following for thematerial:

-   -   α: 35×10⁻⁶ K⁻¹ between 20 and 140° C., and    -   α: 97×10⁻⁶ K⁻¹ above 140° C.

The specific heat Cp is measured by differential enthalpic analysis (DSC30, METTLER) with a temperature rise rate of 10° C./min, for atemperature range varying from 30 to 200° C.

The values of Cp are within the range 1.19 J.g⁻¹.° C.⁻¹ and 1.89 J.g⁻¹.°C.⁻¹ for temperatures between 40° C. and 180° C.

Thermal conductivity measurements are also made for temperatures varyingfrom 25° C. to 180° C. Values are included within the range 0.75 and0.91 W.m⁻¹K⁻¹.

The mechanical properties of the material are also determined bycarrying out compression tests at 23° C. on 10 mm diameter and 20 mmhigh test pieces, using an Adamel Lhomargy DY26 dynamometer and a testspeed of 1 mm/min. The results obtained are as follows:

-   -   compression modulus: 4166±100 MPa,    -   ultimate stress: 155.3±0.8 MPa,    -   compression at failure: 7±0.2%.

Considering the high hydrogen content of the material in example 1, itis particularly suitable for a radiation shielding application.

EXAMPLE 2

The same operating method is used as in example 1, using theconstituents and proportions given in table 1.

The mix also includes:

-   -   0.9% by weight relative to the mass of resin, of the accelerator        NL 49P marketed by Akzo, and    -   1.5% by weight relative to the mass of resin, of the Butanox M50        catalyst marketed by Akzo.

Setting takes place at ambient temperature and after 25 minutes, amaterial with the following characteristics is obtained:

-   -   density: 1.79    -   hydrogen content: 4.80% by weight, namely 5.14×10²² at/cm³,    -   boron content: 0.89% by weight, namely 8.92×10²⁰ at/cm3.

The material obtained has satisfactory thermal properties.

The thermal coefficient of expansion a measured by DSC (METTLER) with atemperature rise of 10° C./min gives the following for the material:

-   -   α: 37×10⁻⁶ K⁻¹ between 20 and 130° C., and    -   α: 109×10⁻⁶ K⁻¹ above 130° C.

The specific heat Cp is measured by differential enthalpic analysis(DSC30, METTLER) with a temperature rise rate of 10° C./min for atemperature range varying from 40° C. to 180° C. Values of Cp are withinthe range 1.07 and 1.65 J.g⁻¹.° C.⁻¹.

Thermal conductivity measurements are also made for temperatures varyingfrom 20° C. to 170° C. Within this temperature range, the value of thethermal conductivity of the resin is close to 0.8 W/m.K.

The mechanical properties of the material are also determined bycarrying out compression tests at 23° C. The compression modulus of thematerial can thus be found, and is equal to 4299±276 MPa.

Given the hydrogen content, the material in example 2 is particularlysuitable for a radiation shielding application.

Thermal aging tests of the material in examples 1 and 2 are also carriedout at 160° C., and on the material in example 1 at 170° C.

Aging tests over 6 months consist in putting samples of the materialwith dimensions 35×25×95 mm into a drying oven at 160° C. and 170° C.and monitoring the mass loss of these samples with time. Variationcurves showing the loss of mass of materials (in %) as a function oftime (in days) are shown in FIG. 1.

Tests were also carried out on the fire reaction of materials inexamples 1 and 2.

Each half-hour fire test at 800° C. was carried out on two 240 mmdiameter and 60 mm high blocks of materials in examples 1 and 2. Theflame was in direct contact with the material for the first blocks,whereas the second blocks were protected by a 1 mm thick steel plate.

In both cases, and for both materials, self-extinguishing occursimmediately after the torch is removed.

EXAMPLE 3

The same operating method is used as in example 1 to prepare a materialfor neutron shielding and maintenance of sub-criticality, from thefollowing mix:

Derakane Momentum 470- 32% by weight 300 vinylester resin zinc borate13% by weight boron carbide B₄C 15% by weight alumina hydrate 40% byweight

The mix also comprises:

-   -   0.9% by weight relative to the mass of resin, of the NL49P        accelerator, and    -   1.5% by weight relative to the mass of resin, of the Butanox M50        catalyst.

Setting takes place at ambient temperature; a material with thefollowing characteristics is obtained after 25 minutes:

-   -   density: 1.8    -   hydrogen content: 4.03% by weight, namely 4.34×10²² at/cm³, and    -   boron content: 13.68% by weight, namely 1.37×10²² at/cm³.

Considering its high boron content, the material in example 3 hasexcellent efficiency in maintaining sub-criticality.

Thus, the material according to the invention has very attractiveproperties for neutron shielding and maintenance of sub-criticality forthe transport of nuclear fuel assemblies.

REFERENCES MENTIONED

-   -   [1] EP-A-0 628 968    -   [2] GB-A-1 049 890    -   [3] JP-A-55 119099

TABLE 1 Example 1 Example 2 Example 3 (% by (% by (% by Constituentsweight) weight) weight) Derakane Momentum 32 32 32 470-300 vinylesterresin Added styrene 5 Zinc borate 6.5 6 13 Zn₂O_(14.5)B₇H₆ Boron carbideB₄C 15 Magnesium hydroxide 56.5 Alumina hydrate 62 40 Gel time 22 min 25min 25 min

1. Composite material for neutron shielding and maintenance ofsub-criticality comprising: (a) a matrix based on vinylester resincomprising at least one compound chosen from the group consisting ofepoxyacrylate vinylester resins, epoxymethacrylate vinylester resins,bisphenol A type vinylester resins, novolac type vinylester resins,halogenated vinylester resins based on bisphenol A, and vinylesterresins obtained from isophthalic polyester and urethane; and (b) aninorganic filler capable of slowing and absorbing neutrons, theinorganic filler comprising at least one inorganic compound of boron andat least one hydrogenated inorganic compound.
 2. Material according toclaim 1 in which the vinylester resin is an epoxy(meth)acrylatebisphenol A type resin complying with the following formula:

in which R represents H or CH₃.
 3. Material according to claim 1 inwhich the vinylester resin is a novolac resin of formula:

in which R represents H or CH₃.
 4. Material according to claim 1 inwhich the inorganic compound of boron is chosen from the groupconsisting of boric acid H₃BO₃, zinc borates Zn₂O_(14.5)H₇B₆, Zn₄O₈B₂H₂and Zn₂O₁₄B₆, colemanite Ca₂O₁₄B₆H₁₀, boron carbide B₄C, boron nitrideBN and boron oxide B₂O₃.
 5. Material according to claim 1 comprising atleast one boron compound chosen among the group consisting of zincborate Zn₂O_(14.5)H₇B₆, and born carbide B₄C.
 6. Material according toclaim 1 in which the hydrogenated inorganic compound is chosen from thegroup consisting of alumina hydrates and magnesium hydroxide. 7.Material according to claim 1 in which the quantities of inorganichydrogenated compound and inorganic compound of boron are such that theboron concentration in the material is equal to 8×10²² to 15×10²¹ ofboron atoms per cm³ and that the hydrogen concentration is 4×10²² to6×10²² atoms per cm³.
 8. Material according to claim 1, comprising 25 to40% by weight of vinylester resin.
 9. Material according to claim 1,with a density equal to or greater than 1.6, preferably 1.65 to 1.9. 10.Material according to claim 1, which can resist a minimum usagetemperature of 160° C.
 11. Process for preparation of a compositematerial according to claim 1, including the following steps: prepare amix of vinylester resin in solution in a vinyl thinner with theinorganic filler, add a catalyst and an accelerator for hardening to themix, degas the mix under a vacuum, pour the resulting mix in a mould,and allow the resulting mix to set in the mould.
 12. Process accordingto claim 11, in which the vinyl thinner is styrene.
 13. Processaccording to claim 11, in which the mould is a transport and/or storagepackaging for radioactive products.
 14. Transport and/or storagepackaging for radioactive products comprising a shield formed from acomposite material according to claim 1.